FIG. 1 shows an exemplary high-RPM disk drive 100 having a magnetic read/write head (or a recording slider) 101 that includes, for example, a tunnel-valve read sensor, that is positioned over a selected track on a magnetic disk 102 using, for example, a two-stage servo system for reading data stored on disk 102. The two-stage servo system includes a voice-coil motor (VCM) 104 for coarse positioning a read/write head suspension 105 and may include a microactuator, or micropositioner, for fine positioning read/write head 101 over the selected track.
A problem associated with tunnel-valve read sensors is that achievable values of the resistance-area (RA) product for tunnel junctions having large and/or optimized Tunnel Magneto-Resistance (TMR) values (i.e., ΔR/R0) are too large for achieving a desirable device resistance RD of less than approximately 300 Ohms for device areas AD that are smaller than 0.1 μm2. The motivation for a lower device resistance RD is primarily for increasing the signal power (∝(ΔR/R)2(Vbias)2/RD) while simultaneously reducing the shot-noise of a tunnel-valve head (∝RD) when operated at a given bias voltage Vbias, which is otherwise limited by additional considerations that are described below. A secondary consideration is for avoiding excessive device impedance mismatch with the characteristic impedance Z0 of the transmission line interconnection to the Arm Electronics (AE) module, which is typically less than or equal to 100 Ohms. A mismatch has the effect of increasing amplifier noise. Accordingly, constraining RD=(RA)/AD to be less than 300 Ohms requires RA to be greater than 1–2 Ohms-μm2.
FIG. 2 is a graph 200 showing the approximate relationship between RA and ΔR/R0 for a typical tunnel junction in which RA and ΔR/R0 are respectively the abscissa and the ordinate of graph 200. As RA is reduced below a “corner” value of RAc by reducing the physical barrier thickness, the low-voltage TMR ratio ΔR/R0 begins to degrade approximately linearly as RA decreases. Tunnel valve barriers typically exhibit an RAc value of approximately 5–10 Ohms-μm2. Thus, the achievable ΔR/R0 for an RA<1–2 Ohms-μm2 will be significantly below the maximum value of ΔR/Rmax that is obtainable for thicker, higher-RA barriers of the same barrier material. Reducing RA by decreasing barrier thickness also results in barriers that are less physically robust and that are more susceptible to pinholes and/or other run-to-run variabilities that can yield large distribution of both ΔR/R0 and RA values across a wafer and/or from wafer-to-wafer. Such variations are much less prevalent when thicker tunneling barriers having RA≧RAC are used.
Additionally, it is well known that the TMR ratio is not independent of the bias voltage, but instead decreases monotonically with larger Vbias. FIG. 3 is a graph 300 showing a typical ΔR/R for a tunneling barrier as a function of Vbias. As shown in FIG. 3, ΔR/R decreases approximately linearly with increasing Vbias≦V50, in which V50 is the value of Vbias for which the TMR ratio ΔR/R has degraded to one-half of its low voltage limit. For this reason alone, it becomes impractical to operate tunnel-valve read sensors at bias voltages larger than Vbias. Long-term degradation, however, usually limits the practical barrier bias voltage Vbias to well below the V50 value. Depending on the barrier material, V50 also tends to degrade for thinner, lower-RA barriers, and is, at best, approximately constant with RA≦RAc.
What is needed is a technique that minimizes the effective device resistance RD of a tunnel valve read head and improves the device Signal-to-Noise Ratio (SNR) of a tunnel valve read head.